The present invention relates to a structure of a semiconductor apparatus having semiconductor circuits made of semiconductor devices such as insulated gate transistors, and also to a method of manufacture thereof. Particularly, the invention relates to a technology that forms a crystalline semiconductor film over an insulating surface. The semiconductor apparatus include not only semiconductor devices such as thin-film transistors (TFTs) and MOS transistors but also displays and electrooptic apparatus, such as image sensors, both of which have semiconductor circuits made of insulated gate transistors. In addition, the semiconductor apparatus of this invention also include electronic apparatus incorporating these displays and electrooptic apparatus.
An active matrix liquid crystal display, which forms a pixel matrix circuit and a drive circuit by using thin-film transistors (TFTS) formed on an insulating substrate, is attracting attention. A liquid crystal display currently used as a monitor has a size range of 0.5-20 inches.
TFTs that use as an active layer a crystalline semiconductor film represented by polysilicon are being spotlighted as a means for realizing the liquid crystal display capable of displaying finely defined images. Although TFTs using a crystalline semiconductor film as an active layer have a faster operating speed and a higher driving capability than those TFTs that use an amorphous semiconductor film as an active layer, their TFT characteristics are difficult to control.
One of the causes for the difficulty in controlling the TFT characteristics is the property of an interface between the active layer and the insulating film. This interface, when contaminated, makes it difficult to manufacture the semiconductor devices with good controllability of the TFT characteristics. It is therefore important to clean the interface between the active layer and the insulating film.
Currently, there are growing demands on the TFTs for high mobility and it is considered more promising to use a crystalline semiconductor film with high mobility than to use an amorphous semiconductor film as the TFT""s active layer. The method of manufacturing a conventional top gate type TFT using a crystalline semiconductor film will be briefly explained.
First, a substrate having an insulating surface is formed with a base insulating film (hereinafter referred to as a base film) and then heat-treated, after which it is deposited with an amorphous silicon film. Next, the amorphous silicon film is subjected to crystallization processing such as heating and laser beam irradiation to form a polysilicon film (polycrystalline silicon film). Next, the polysilicon film is patterned to a desired shape and deposited with an insulating film (gate insulating layer) and a conductive film (gate line forming material layer). Then these films are patterned to form gate lines. Next, impurities of p- or n-type conductivity are selectively introduced into the polysilicon film to form impurity regions, such as source and drain regions. This is followed by depositing an interlayer insulating film, forming contact holes to expose the source and drain regions, forming a metal film, and then patterning the metal film to form metal lines in contact with the source and drain regions. In this way, the process of manufacturing the TFTs is completed.
In the conventional technology described above, when the base film after being formed is subjected to the heat treatment to improve the TFT reliability, the surface of the base film is exposed to the atmosphere. At this time, the base film surface is contaminated with impurities contained in the air (boron, oxygen, water, sodium, etc.). If a semiconductor film that constitutes an active layer is formed over and in contact with the base film contaminated by the open air, the characteristic of an interface between the active layer, particularly a channel forming region, and the base film deteriorates, leading to a degradation in the electrical characteristic of TFTs.
Because the air in the clean room contains boron from a HEPA filter generally used for cleaning, an unspecified amount of boron mixes into the surface of the film exposed to the atmosphere. The HEPA filter is made of a mesh of glass used to remove minute particles in the air. The glass contains a large amount of boron to make it easy to manufacture the mesh-like structure. Using other filters than the HEPA filter is disadvantageous from the standpoint of reducing the manufacturing cost.
To investigate the effect of the impurities in the air, the insulating base film after being formed is exposed to the atmosphere and then deposited with a laminated structure of semiconductor films made of amorphous silicon films to form TFTs. The SIMS analysis on the manufactured TFTs revealed a concentration peak of boron whose maximum value was 3xc3x971017 atoms/cm3. When boron mixes into the active layer of semiconductor film, it is diffused and activated by the processes that follow (heat treatment and laser beam processing, etc.) making the control of the impurity concentrations in the active layer difficult. Measurement of the TFT electric characteristic revealed a phenomenon in which the threshold voltage shifts to the plus side.
When impurities (boron, oxygen, water, sodium, etc.) enter into the active layer of semiconductor film, it is found that the crystallization of the semiconductor film is hindered.
The present invention provides a semiconductor apparatus having semiconductor circuits made of semiconductor devices which improve the interface between an active layer, particularly a channel forming region, and a base film to improve the TFT characteristic (such as threshold voltage) and which have high reliability. The invention also provides a method of manufacturing such semiconductor apparatus.
To achieve the above objectives, this invention is characterized in that the first base film, after it has been formed, is heat-treated and then the second base film (an insulating film having a thickness smaller than that of the first base film) and the semiconductor film are successively formed in laminated layers without being exposed to the atmosphere. This arrangement prevents the active layer, particularly the interface between the channel forming region and the second base film, from being contaminated, thus realizing stable and good electrical characteristics.
A first aspect of the invention disclosed in this specification is a semiconductor apparatus having a semiconductor circuit made of semiconductor devices, which semiconductor apparatus comprises: a first insulating film formed on a substrate; a second insulating film in contact with the first insulating film; a channel forming region and source and drain regions formed on both sides of the channel forming region, the channel forming region and the source and drain regions being formed in contact with the second insulating film and; a gate insulating layer in contact with the channel forming region; and a gate line provided over the channel forming region with the gate insulating layer interposed therebetween; wherein the second insulating film is thinner than the first insulating film.
In this configuration, an impurity concentration in an interface between the first insulating film and the second insulating film is higher than an impurity concentration in an interface between the second insulating film and the channel forming region.
Further, in the above configuration, the second insulating film and the channel forming region are formed by at least a step of successively forming them in laminated layers without exposing them to the atmosphere.
Further, in the above configuration, the first insulating film is formed by at least a heat treatment step.
Further, in the above configuration, the first insulating film has a thickness of 100-500 nm.
Further, in the above configuration, the second insulating film has a thickness of 10-100 nm.
Further, in the above configuration, the second insulating film is a single-layer film of selected one of silicon nitride film, silicon oxynitride film and silicon oxide film, or a laminated film of these films.
Further, in the above configuration, a low concentration impurity region is provided at least between the channel forming region and the source region or between the channel forming region and the drain region.
Further, in the above configuration, a catalytic element that accelerates crystallization of silicon is contained in at least the source region and the drain region.
Further, the catalytic element is at least one or more elements selected from Ni, Fe, Co, Pt, Cu, Au and Ge.
In this specification, the xe2x80x9camorphous semiconductor filmxe2x80x9d denotes typically a semiconductor film having an amorphous material, such as amorphous semiconductor film having microcrystals and microcrystalline semiconductor film. These semiconductor films are formed of Si film, Ge film or a compound semiconductor film [for example, amorphous silicon germanium film expressed by SiXGe1xe2x88x92X (0 less than X less than 1)]. The semiconductor film can be formed by a known technology, such as reduced pressure thermal CVD method, thermal CVD method and PCVD method.
In this specification, the xe2x80x9ccrystalline semiconductor filmxe2x80x9d denotes a single-crystal semiconductor film and a semiconductor film containing grain boundary (including polysilicon semiconductor film and microcrystalline semiconductor film). It is clearly distinguished from a semiconductor film which is amorphous in its entire area (amorphous semiconductor film). It is needless to say that in this specification the word xe2x80x9csemiconductor filmxe2x80x9d, of course, includes an amorphous semiconductor film as well as a crystalline semiconductor film.
Further, in this specification the xe2x80x9csemiconductor devicexe2x80x9d denotes a switching device and a memory element, such as a thin-film transistor (TFT) and a thin-film diode (TFD).
A first method of manufacturing a semiconductor apparatus according to this invention is a method of manufacturing a semiconductor apparatus having a semiconductor circuit made of semiconductor devices, which comprises the steps of: forming a first insulating film over a substrate; heat-treating the first insulating film; successively forming over the first insulating film a second insulating film and a semiconductor film in laminated layers without exposing them to the atmosphere; and crystallizing the semiconductor film to form a crystalline semiconductor film.
A second method of manufacturing a semiconductor apparatus according to this invention is a method of manufacturing a semiconductor apparatus having a semiconductor circuit made of semiconductor devices, which comprises the steps of: forming a first insulating film over a substrate; heat-treating the first insulating film; successively forming over the first insulating film a second insulating film and a semiconductor film in laminated layers without exposing them to the atmosphere; introducing into at least a part of the semiconductor film a catalytic element for accelerating crystallization; and crystallizing the semiconductor film to form a crystalline semiconductor film.
A third method of manufacturing a semiconductor apparatus according to this invention is a method of manufacturing a semiconductor apparatus having a semiconductor circuit made of semiconductor devices, which comprises the steps of: forming a first insulating film over a substrate; heat-treating the first insulating film; successively forming over the first insulating film a second insulating film and a semiconductor film in laminated layers without exposing them to the atmosphere; introducing into at least a part of the semiconductor film a catalytic element for accelerating crystallization; crystallizing the semiconductor film to form a crystalline semiconductor film; and removing the catalytic element by gettering.
In one of the first to third manufacturing method, the second insulating film is formed smaller in thickness than the first insulating film.
Further, in one of the first to third manufacturing method, the first insulating film is heat-treated at a temperature of 200-700xc2x0 C.